Cellulose Nanofiber (CNF) is the world’s most advanced biomass material, made from wood-derived fiber (pulp) that has been micro-refined to the nano level of several hundredths of a micron and smaller.
Plant-derived materials, such as cellulose, are frequently thermally insulating. A new material made from nanoscale cellulose fibers exhibits high thermal conductivity in the opposite direction. This makes it useful in areas where synthetic polymer materials were previously dominant. Because cellulose-based materials have environmental advantages over polymers, research into this area could lead to greener technological applications requiring thermal conductivity.
Cellulose is a structural component of plant cell walls that allows trees to reach such heights. The secret to its material strength, however, lies in its overlapping nanoscopic fibers. Many commercial products have used cellulose nanofiber (CNF) materials in recent years because their strength and durability make them a good replacement for polymer-based materials such as plastics, which can be harmful to the environment.
Our main challenge was how to measure the thermal conductivity of such small physical samples and with great accuracy. We used a technique known as T-type thermal conductivity measurement for this. It enabled us to measure the thermal conductivity of rod-shaped CNF yarn samples that are only micrometers in diameter.
Professor Junichiro Shiomi
However, for the first time, a research team led by Professor Junichiro Shiomi of the University of Tokyo’s Graduate School of Engineering investigated previously unknown thermal properties of CNF, and their findings indicate that these materials could be even more useful. Their findings were published in the journal Nano Letters.
“If you see plant-derived materials such as cellulose or woody biomass used in applications, it’s typically mechanical or thermally insulating properties that are being employed,” said Shiomi. “When we explored the thermal properties of a yarn made from CNF, however, we found that they show a different kind of thermal behavior, thermal conduction, and it’s very significant, around 100 times higher than that of typical woody biomass or cellulose paper.”
The reason yarn made from CNF can conduct heat so well is due to the way it’s made. Cellulose fibers in nature are very disorganized, but a process called the flow-focusing method combines cellulose fibers, orientating them in the same way, to create CNF. It’s this tightly bound and aligned bundle of rod-shaped fibers that allows heat to transfer along the bundle, whereas in a more chaotic structure it would dissipate heat more readily.
“Our main challenge was how to measure the thermal conductivity of such small physical samples and with great accuracy,” said Shiomi.
“We used a technique known as T-type thermal conductivity measurement for this. It enabled us to measure the thermal conductivity of rod-shaped CNF yarn samples that are only micrometers (one thousandth of a millimeter) in diameter. However, the next step for us is to conduct precise thermal tests on two-dimensional textile-like samples.”
Shiomi and his colleagues hope that their research and future investigations into the use of CNF as a thermally conductive material will provide engineers with an alternative to some environmentally harmful polymers. Because of the biodegradable nature of CNF and other plant-based materials, it could greatly reduce the consequences of discarded electronic equipment, or e-waste, in applications where heat transfer is important, such as certain electronic or computational components.
